1. Field of Invention
The invention pertains to wavelength-agile optical filters suitable for wavelength-division-multiplexed (WDM) optical communications networks. More particularly, the invention pertains to optical filters with a wavelength reference that can be remotely switched to arbitrarily selectable channels on a standard grid, and to re-configurable optical communications networks employing same.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
The T-1 standards committee ANSI has provided a draft document, “ANSI T1.105-1988”, dated Mar. 10, 1988, which sets forth specifications for rate and format of signals which are to be used in optical interfaces. The provided specifications detail the Synchronous Optical Network (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level time division multiplexed public switched telephone network (PSTN) signals into a common medium. In essence, SONET established a uniform standardization transmission and signaling scheme, which provided a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
Currently this expansion of bandwidth is being accomplished by what is known as “wavelength division multiplexing” (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions of the light spectrum. Therefore, WDM is the optical equivalent of frequency division multiplexing (FDM). Current International Telecommunications Union (ITU) specifications call for channel separations of approximately 0.4 mm, i.e., 50 GigaHertz. At this separation, as many as 128 channels may be carried by a single fiber in a bandwidth range within the same capacity of an erbium doped fiber amplifier (EDFA).
Across an optical network a broad range of optical components, e.g. transmitters, receivers, multiplexers, demultiplexers, amplifiers, isolators, etc. must be fabricated to handle each of these channels. Typically each component is fabricated for a specific channel.
For optical transmitters, for example, each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission. The lasers presently deployed are distributed-feedback (DFB) diode lasers. The wavelength grid is defined by the transmission peaks of a Fabry-Perot reference etalon. The technology for fabricating these etalons from glass or fused silica in large quantities at reasonable cost is well developed. Present transmitters use a combination of a DFB laser and a reference etalon in a feedback control loop. DFB lasers suffer from the drawback that they have small tuning ranges of ˜3 nm (See, e.g., J. Carroll, J. Whiteaway, & D. Plumb, Distributed feedback semiconductor lasers, SPIE Press, 1998). A DFB laser's characteristics can only be approximately determined by design. In practice the center wavelengths are distributed across the grid span due to statistical variations in the manufacturing process. Thus each DFB laser transmitter must be individually sorted and is usable for only one channel or a small number of adjacent channels. As a result, DFB laser transmitters cannot be assigned to arbitrary channels.
On the other hand, WDM networks are evolving towards re-configurable architectures in which each transmitter's wavelength must be re-selectable by remote command. Re-configurable networks offer significant capacity, reliability, and management advantages over static systems (See, e.g., R. Ramaswami and K. Sivarajan, Optical Networks, A Practical Perspective, Morgan Kaufmann Publishers, 1998).
Thus there exists a need for optical components including transmitters that can be arbitrarily switched to desired channel wavelengths within the wavelength grid of a WDM network.